The present invention relates to a radio frequency (RF) magnetic shield for magnetic resonance imaging (MRI) disposed between gradient coils and an RF coil in an MRI apparatus, and more particularly to the shield having a cylindrical conductive plate in which slits are formed for preventing eddy currents due to gradient fields from occurring and the respective slits are electrically connected through capacitors for maintaining the high effect of RF magnetic shielding.
A typical MRI apparatus has an allocation of coils shown in FIG. 1. The MRI apparatus shown therein adopts a superconductive magnet 1 having a cylindrical cryostat 2. A superconductive main coil 3 is disposed in the cryostat 2. In a bore of the cryostat 2, there disposed are a shim coil 4, a gradient coil 5, and an RF coil 6 which are arranged in this order from the inner wall. A cylindrical RF magnetic shield 7, which is made of copper foil, is disposed between the gradient coil 5 and the RF coil 6, with the shield 7 attached to the inner wall of the gradient coil 5.
The above RF magnetic shield 7 should satisfy three requirements. The first is to shield the RF coil 6 from RF noises transmitting through the gradient coil 5. Here, noises at resonance frequencies such as around 63.9 MHz (for magnetic strength of 1.5 tesla) are our interest (in particular, their magnetic components) and are regarded as the RF noises. Second, it is required that magnetic coupling between the gradient coil 5 and the RF coil 6 be removed to prevent a value of Q of the RF coil 6 from being lowered. These first and second requirements should be satisfied by a conductive, non-magnetic metal plate having larger shielding area and having no holes or the like therethrough.
Third, it is required for the shield 7 that, when a gradient field is generated from the gradient coil 5, eddy currents of lower frequencies (approx. tens of kHz or less) induced in the shield 7 by the gradient field be suppressed to remove image distortion resulted from rounded wave forms of the gradient field. In order to prevent the lower frequency eddy currents from generating, the RF magnetic shield 7 has a plurality of slits through its copper foil which are spaced to each other and are shaped into appropriate given sizes.
However, the slits for getting rid of the eddy currents of the lower frequencies degrade the RF shielding effect for the foregoing first and second requirements, since eddy currents of higher frequencies, different from the above lower frequency ones and used to offset the magnetic field of the RF noises (63.9 MHz, for instance) by generating a magnetic field in an opposite direction, are weakend by the slits lying against their flow passes. In short, the first and second requirements are in conflict with the third requirements in regard to satisfying them at the same time. Therefore, it had been long felt that another measure for satisfying the first to third requirements at the same time were required.
For this purpose, a technique shown in FIG. 2 is proposed (refer to "Shield for Decoupling rf and Gradient Coils in an NMR Apparatus" by K. Yoda, Japanese Journal of Magnetic Resonance in Medicine", Vol.9 No.1(1989), Pages 86 to 89). The shield shown therein has a conductive plate consisting of a plurality of long, conductive plates 7a, . . . ,7a. Two adjacent plates 7a and 7a are overlapped to each other, and long capacitors are formed between the long overlapped parts of the plates 7a and 7a by inserting dielectrics 8, . . . ,8, respectively, thus the two adjacent plates 7a , . . . ,7a being connected with each other through the distributed-constant-type capacitor. In addition, lumped-constant-type chip capacitors 9, . . . ,9 are in parallel connected, between adjacent two plates 7a and 7a, with the distributed-constant-type capacitor. The chip capacitors 9 , . . . ,9 are for increasing capacitance between two adjacent plates 7a and 7a.
The impedance Z at the joint between two adjacent plates 7a and 7a is equivalent to a parallel network of a resistor R and a capacitor C shown in FIG. 3. When .omega. is an angular frequency, the impedance Z is represented by the following equation. EQU .vertline.Z.vertline.=R/(1+.omega..sup.2.R.C).sup.1/2
As apparent from the equation, the capacitors according to the overlapping decrease the impedances at the respective plate joints in order to prevent eddy currents from being cut off, which generate the magnetic field for cancelling the RF magnetic field, as stated above.
However, there is a drawback in the shield of FIG. 2. The structure of each of the Joints is referred to as a monolayer capacitor (i.e., parallel-plate capacitor) using the overlapping of plates. In FIG. 4 showing the monolayer capacitor a reference numeral 7aa represents an adhesive layer, not counted as layers. There exists a physical limitation in increasing the area of pole plates (namely, the overlapped area). Also, taking reliability of insulation into consideration, there is a certain minimum in the distance between two pole plates (namely, the conductive plates 7a and 7a ). Hence, it is in fact impossible to increase largely the capacitances at the plate Joints, thereby the impedances thereat still remaining comparatively large.
Moreover, the auxiliary chip capacitors 9, . . . ,9 are placed at certain selected positions in a longitudinal direction of the overlapped capacitor, not all positions are in the longitudinal direction. Therefore, the attachment of the chip capacitors 9, . . . ,9 is not always effective in reducing capacitance at every part of each of the joints. This results in reduced high-frequency magnetic noises which should be be removed.
There is another drawback in the structure shown in FIG. 2. When the capacitors utilize the overlapping technique as stated, it is necessary to assemble thin conductive foils (pole plates) into the capacitors. Assembling the thin foils is liable to not only deteriorate the accuracy of processing, thus leading to unstable shielding effect, but gives lower efficiency of assembling.